Five years ago we hypothesized that monocytes employ myeloperoxidase (MPO) to generate reactive species that contribute to oxidative stress, cellular injury and conversion of LDL into an atherogenic form. During the conduct of this research we defined novel mechanisms for generation of reactive halogen, nitrogen, and free-radical species by leukocytes via MPO, and the importance of nitric oxide (NO)-peroxidase interactions in modulating both NO bioavailability and function. More recently, efforts have extended to clinical studies aimed at testing the hypothesis that MPO and specific markers of oxidant stress are linked to cardiovascular risk and are modulated by known risk-reducing therapies. Despite the many links between MPO, oxidant stress, and coronary artery disease (CAD), many critical questions remain. For example, direct demonstration of a causal role for the enzyme in disease development and progression remains to be established. Further, the role of oxidation in atherogenesis has recently been questioned based upon the failure of multiple "antioxidant" trials. The present proposal is both an extension of our earlier research, and a direct effort to address these questions. It is predicated upon the hypothesis that MPO and oxidative stress are mechanistically linked to the development of cardiovascular disease. It integrates studies on basic mechanisms with a search for specific reaction products that reveal whether relevant pathways operate in human disease, and in animal models of inflammation. In preliminary studies we present evidence for a novel catalytic activity for MPO that may enable the enzyme to participate in endothelial dysfunction in CAD. We demonstrate pathways through which MPO catalysis leads to formation of reactive aldehydic intermediates that are cytotoxic and implicated in atherogenesis. We demonstrate that MPO and systemic measures of its activity are linked to atherosclerotic risk and oxidant stress in vivo. The overall goals of this proposal are to: (i) provide mechanistic insights into novel pathways of MPO catalysis and function; (ii) test the hypothesis that MPO generates cytotoxic aldehydes that promote cellular injury and oxidant stress in vivo; and (iii) use a combination of genetic and biochemical approaches to define the role of MPO and specific oxidation pathways in cardiovascular disease.